The present invention relates to a modified polyolefine which has good adhesion to metals and to other polar substances.
Polyethylene and polyolefines on the whole are characterized by poor adhesion to metals and to materials containing polar groups. Somewhat better adhesion is achieved in certain instances by copolymerizing unsaturated esters, such as vinylacetate or alkyl(meth)acrylate (methylacrylate, ethylacrylate, butylacrylate, etc.), however these copolymers also present inadequate adhesion in most multi-layer structures.
Various ways have been tried to improve this adhesion. Unsaturated acids or acid anhydrides, among others, have been used as comonomers in polymerizing copolymers or terpolymers of ethylene. Examples of commercial applications are ethylene/acrylic acid copolymer which contains 9% acrylic acid, and ethylene/methacrylic acid copolymer which contains 9% methacrylic acid. A known commercial terpolymer contains 4% acrylic acid and 7% butylacrylate. A product in which the methacrylic acid has partially been neutralized to salt so that a so-called ionomer has been obtained, is also commercially available. An example of the use of maleic acid anhydride towards improving adhesion, is the application in which maleic acid anhydride has been grafted to LDPE, HDPE or EVA. Another example is the terpolymer of ethylene, butylacrylate and maleic acid anhydride.
Direct copolymerizing of ethylene with an unsaturated acid is disadvantageous for several different reasons. The acids are corrosive and reduce the service life of the apparatus both in the synthesis step and in the shaping step. Moreover, these ehtylene/acid copolymers or terpolymers are thermally unstable, with the consequence that low working temperatures have to be employed, with restrictions on production rate and product quality following therefrom. High acid quantities are indispensable for achieving satisfactory adhesion because these copolymerized acids are immobile. It is therefore difficult for these acids to exit from the polymer matrix and to reach the polar groups in the boundary layer. Large acid quantities are also required in order to reduce crystallinity, this being indispensable for achieving adhesion. This may naturally be attained by using a third monomer, such as butylacrylate or vinylacetate for example.
Terpolymers are, however, uneconomical in view of recovery of the comonomers in the synthesis. Therefore, the content of the comonomer which is active regarding adhesion, is limited. The total comonomer content is also limited by laws in various countries regarding foodstuff compatibility. Crystallinity may also be lowered by admixing thermoplastic elastomer, e.g. polyisobutylene (PIB). For instance, attempts have been made when grafting maleic acid anhydride to improve mobility thereof by using spacer groups between the maleic acid anhydride and the polymer (Diels-Alder reaction between diene and maleic acid anhydride, and grafting of the same with the polymer).